1. Field of the Invention
The present invention relates to a power transmission device for transmitting rotational power, which is generated by an engine, from the engine to engine accessories. More particularly, the present invention relates to a power transmission device having a torque limiter mechanism to shut off a power transmission path from the engine to the engine accessories when an overload torque is given to the engine, for example, when a drive shaft of driving the engine accessories is locked for the reason of seizure and so forth.
2. Description of the Related Art
In a conventional refrigerating cycle having a variable displacement type refrigerant compressor, which will be referred to as a compressor hereinafter, in which a discharge capacity of discharging the refrigerant can be changed to 0% of the compressor capacity, it is unnecessary to provide a clutch mechanism which connects and disconnects the transmission of rotational power (torque) from the engine to a drive shaft of the refrigerant compressor. However, in the case where the clutch mechanism is not provided, if the compressor shaft is locked by seizure of the compressor, an overload torque (impact torque), the intensity of which is much higher than the intensity of usual transmission torque, is generated. Due to the above overload torque, the rotation of a V-belt pulley to drive the compressor shaft is stopped. Accordingly, the V-belt driven by the engine slips, which causes abrasion of the V-belt. Further, the V-belt is heated. As a result, there is a possibility that the V-belt is broken.
In order to solve the above problems, a compressor-pulley device is proposed, for example, in Japanese Unexamined Patent Publication No. 2001-173759, by which a power transmission path from the engine to the compressor is shut off when a difference in torque between the pulley and the compressor shaft exceeds a setting torque when an overload torque is generated for the reason of lock of the compressor shaft. As shown in FIG. 4, this torque limiter is composed in such a manner that rotational power of the engine transmitted from the engine to the compressor shaft is transmitted from the pulley (not shown) to the compressor shaft 103 via the rubber damper (not shown), the outer hub 101 made of resin and the inner hub 102 made of metal.
A connecting portion of connecting this inner hub 102 with the shaft 103 is composed in such a manner that the male screw portion 104 provided on the outer circumference of the forward end portion of the compressor shaft 103 is screwed to the female screw portion 106 provided in the cylindrical boss portion 105 of the inner hub 102 which is formed by means of insert molding on the inner circumferential side of the outer hub 101 of the compressor-pulley device. This inner hub 102 is made of metal and is formed by means of integral molding. In the case where an excessively high torque is given when the compressor shaft is locked, the bridge portion 107 of the inner hub 102 is broken. In this way, the torque limiter mechanism fulfills its function.
In this structure, the shoulder portion front end face 111 of the compressor shaft 103 and the boss portion rear end face 112 of the inner hub 102 come into face-contact with each other in which the contact face is formed into an annular shape, and this contact portion composes a stopper. An axial fastening force of fastening the male screw portion 104 to the female screw portion 106, which is generated by the rotational power of the engine and transmitted through the pulley, is received by the stopper face. Due to this structure, the inner circumference of the boss portion 105 of the inner hub 102 can be fastened to the outer circumference of the forward end portion of the compressor shaft 103 by a predetermined fastening axial force (fastening torque). Therefore, even when the rotation fluctuates, the occurrence of rattling in the rotational direction can be avoided. Accordingly, reliability of the compressor-pulley device can be enhanced.
In this connection, from the viewpoint of global environmental protection, they have been recently developing a refrigerating cycle of the air-conditioner for vehicle use in which carbon dioxide (CO2) is used as the refrigerant. Compared with a refrigerant such as fluorocarbon and chlorofluorocarbon, the working pressure of carbon dioxide is high. Therefore, even in the case of equalization pressure in which the refrigerating cycle is not used, pressure of this refrigerant of carbon dioxide is high. In the case of an open type compressor, the shaft is given a force by the inner pressure so that the shaft is pushed outside from the front wall face of the housing. Accordingly, when CO2 is used for the refrigerating cycle in which the compressor-pulley device having the torque limiter mechanism is used, a force given to the shaft in the direction of the outside of the compressor becomes stronger than that in the case of the conventional fluorocarbon or chlorofluorocarbon. Accordingly, a power loss caused in the thrust bearing, which is built in the compressor, is increased.
A shaft seal conducts sealing while the shaft seal squeezes a sliding face. This squeezing force is increased when the inner pressure increases. Therefore, the power loss is also increased in this case. Accordingly, the power loss in the case of OFF-operation of the refrigerating cycle is increased. In order to reduce the power loss described above, an outer diameter of the shaft of the compressor may be reduced. When the outer diameter of the shaft of the compressor is reduced, a sectional area of the shaft is decreased, and a force which pushes the shaft in the direction of the outside of the housing, is reduced, so that a load given to the thrust bearing can be decreased. Further, a contact area and contact length of the shaft seal can be reduced, so that the power loss can be decreased.
However, when the outer diameter of the compressor shaft is reduced in the constitution of the prior art, sizes of the male and the female screw portion in the connecting portion of connecting the inner hub with the shaft are decreased, and the mechanical strength of the male and the female screw portion becomes insufficient with respect to the fastening axial force. Further, a contact area of the stopper between the shaft and the inner hub is reduced, and buckling may be caused. For the above reasons, it is not appropriate that the outer diameter of the compressor shaft is simply reduced. On the other hand, it is possible to think that the mechanical strength of material composing the inner hub is increased, however, the inner hub is formed by a mechanical strength in which consideration is given to the torque limiter mechanism. Therefore, it is impossible to adopt the aforementioned structure.